激光冲击诱导弹簧钢丝残余应力状态分析

利用高功率Nd:YAG激光对不同工艺处理的SWOSC-V弹簧钢丝进行单点冲击处理,用X射线应力分析仪测量弹簧内外侧、侧表面的残余应力并计算出残余主应力,建立了激光冲击SWOSC-V弹簧钢丝表面残余应力的产生模型,并利用该模型分析了弹簧钢丝表面残余应力产生的原因。结果表明:弹簧钢丝在经激光冲击处理的表面强化区产生残余压应力,钢丝退火后直接激光冲击处理与经喷丸强化的钢丝激光冲击处理的表面残余应力变化不同,喷丸强化所引起的材料硬化是激光冲击处理弹簧钢丝残余应力变化不同的原因。     

Application of interferometric strain rosette to residual stress measurements

An interferometric strain rosette (ISR) technique is extended to residual stress measurements. The ISR technique is based on diffraction and interference of laser light reflected from three micro-indentations depressed in a specimen surface. Three in-plane strain components between the three micro-indentations can be measured simultaneously. Therefore the ISR enables a determination of two normal and one shear strain components. For many applications, the ISR is superior to a resistance strain rosette due to its short gage length and non-contacting nature. By applying an ISR to a material surface, residual stresses at the location of the ISR can be obtained through measurement of residual strains relieved via hole-drilling. Since the gage length can be as short as 50 μm, the ISR is capable of recording high strain gradients and it allows the strains close to the hole to be measured. The size of the hole can be small and precise location is not required. Since the ISR technique is non-contacting, it may be used to measure residual stresses in hostile environments.     

A numerical study of residual stress induced in machined silicon surfaces by molecular dynamics simulation

Residual stresses in machined surface are regarded as a critical factor affecting the quality and service life of components. However, little research has been conducted to reveal the formation of residual stresses as well as the relation between machining conditions and residual stresses at the nanometric scale. In this study, residual stresses in machined surfaces of monocrystalline silicon are computed based on molecular dynamics simulation. An orthogonal machining configuration is adopted, and diamond cutting tools are used. The numerical approach developed is able to reveal stress evolution during and after machining, as well as in-depth residual stress distributions. The results indicate that the material stresses are stabilized within a manageable amount of computation time, and the in-depth normal stress along the tool moving direction has a more dynamical and significant pattern compared with other stress components. Meanwhile, the effects of depth of cut and tool rake angle are investigated. It is found that the increase of depth of cut results in the decrease of maximum tensile residual stress on the machined surfaces and the increase of maximum compressive residual stress underneath the surface. Similar observations are observed when the tool rake angle changes from positive to negative. It is believed that the more negative tool rake angles or the larger depths of cut induce a more drastic phase transformation to the machined surfaces, and this makes the in-depth residual stress distributions more compressive.     

Modeling of elastic wave propagation on a curved free surface using an improved finite-difference algorithm

Modelling and understanding the effect of elastic wave propagation along a curved free surface has been one of the important issues in seismic exploration[1—3], earth seis-mology[4], and non-destructive ultrasonic detection[5]. Several approaches have beenproposed for simulating wave propagation in heterogeneous media with a topographic stress-release boundary. These include finite-element methods (FEM), boundary element methods (BEM), finite-difference methods (FDM), pseudo-spectral metho…     

Abnormal residual stress in nanostructured Al thin films grown on Ti/glass substrates

The residual stress and micro-structural properties of nanostructured Al thin films prepared by electron beam evaporator are studied. The films were grown on Ti/glass substrates at normal and oblique angles of inclination. The average aspect ratio of Al nanorods produced at an oblique angle of incidence of 85°, increased from 2.2 to 6.0, as the thickness of the films increased from 100 nm to 600 nm. The column tilt angle of Al nanorods was observed to be in close agreement with the theoretical value. The XRD pattern of nanostructured Al thin films showed (111) planes oriented parallel to the substrate surface. The crystallite size was observed to be ～9 nm for all the films produced at oblique angle deposition (OAD). Abnormal residual stresses were determined in the films produced at OAD. The nanocrystalline films produced at normal angle, exhibited tensile residual stress, while, the residual stresses in the films produced at oblique angles of inclination (α = 65°, 75°), were observed to be compressive. Residual stress-free nanocolumnar Al films (Al nanorod films) were observed, when they were grown at an oblique angle of inclination of 85°.     

Mechanical stresses in an irradiated target with a disturbed surface

Mechanical stresses generated in a disturbed-surface solid target irradiated by intense fluxes of accelerated charged particles are studied numerically and analytically. The disturbed surface is needed to analyze the influence of microirregularities present on real surfaces on the stress pattern. Three basic components of the stress field are revealed: a shock wave; stresses localized in the energy liberation region; and disturbed stresses, which are due to the disturbed surface. The disturbed stresses are localized in a surface layer of width comparable to the disturbance wavelength and account for about 30% of the field stress energy. It is concluded that the disturbance of the surface and disturbance-induced stresses should be taken into account in analysis of structural transformations under irradiation.     

Study on stress birefringence measurement of uniaxial crystal

Residual stress birefringence in crystal will affect frequency conversion efficiency and beam quality. In this paper the distribution characteristic of inherent stress birefringence in uniaxial crystal is analyzed, qualitative results are obtained for KDP crystals by digital stress measurement instrument, and the stress gradient distribution is calculated, also the effect of deviation from optical axis on the measured stress distribution results is discussed. Exact determination of stress birefringence and its spatial distribution is of great importance to the manufacture and application of optical materials and components.     

A micro-scale strain rosette for residual stress measurement by SEM Moiré method

In this paper,a new method combining focused ion beam(FIB)and scanning electron microscope(SEM)Moirétechnique for the measurement of residual stress at micro scale is proposed.The FIB is employed to introduce stress relief like the macro ring-core method and fabricate gratings with a frequency of 5000 lines/mm on the measured area of the sample surface.Three groups of gratings in different radial directions are manufactured in order to form a micro-scale strain rosette.After milling ring-core by FIB,the deformation incurred by relief of the stress will be recorded with the strain rosette.The displacement/strain field can be measured using SEM scanning Moiréwith random phase-shifting algorithm.In this study,the Nickel alloy GH4169 sample(which was processed by laser shock peening)is selected as a study object to determine its residual stress.The results showed that the components of the in-plane principal stresses were-359 MPa and-207 MPa,respectively,which show good agreement with the results obtained from the available literature.     

2024铝合金激光多次冲击诱导残余应力状态分布规律

采用高功率激光器多次冲击2024铝合金,用X射线衍射技术分析了冲击区域的残余应力,研究了冲击残余应力状态分布规律,并用其评价激光冲击强化效果。研究表明,随着冲击次数增加,塑变量及塑性应变梯度逐渐减小,测点是双向压应力状态,而4次冲击时,塑性应变梯度增大,光斑中心是单向压应力状态,其他点是双向压应力状态。当激光功率密度为2.8 GW/cm2时,3次冲击强化效果最佳,材料是二向压应力状态,残余最大主应力及应力强度的均值最大,方差最小,分布基本均匀,塑性应变梯度较小。     

Dislocation substructures and internal stress fields in bulk- and differentially quenched rails

A layer-by-layer analysis of rails bulk-hardened in oil and differentially hardened in a variety of regimes is performed by means of transmission electron microscopy. Quantitative parameters of dislocation substructures and internal stress fields, and their dependences on the distance from the tread contact surface are established. It is shown that the most dangerous stress concentrators are interfaces between globular cementite matrix particles; such interfaces form predominantly in rails subjected to bulk quenching.     

Influence of Ytterbia Content on Residual Stress and Microstructure of Y2O3--ZrO2 Thin Films Prepared by EB-PVD

Y2O3 stabilized ZrO2 （YSZ） thin films with different Y2O3 molar contents （0, 3, 7, and 12 mol%） are deposited on BK7 substrates by electron-beam evaporation technique. The effects of different Y2O3 contents on residual stresses and structures of YSZ thin films are studied. Residual stresses are investigated by means of two different techniques： the curvature measurement and x-ray diffraction method. It is found that the evolution of residual stresses of YSZ thin films by the two different methods is consistent. Residual stresses of films transform from compressive stress into tensile stress and the tensile stress increases monotonically with the increase of Y2O3 content. At the same time, the structures of these films change from the mixture of amorphous and monoclinic phases into high temperature cubic phase. The variations of residual stress correspond to the evolution of structures induced by adding of Y2O3 content.     

A role of fringe pattern catalogue in the course of interferometrically based determination of residual stresses by the hole-drilling method

A further development of the technique for residual stresses determination in thick-walled structures, which is based on a combination of the hole-drilling method and reflection hologram interferometry, is presented. A plane specimen welded from two equal parts of dimensions 130×80 mm² in plane and thickness 12 mm is the object of investigation. Weld seam is performed along the shortest side of the specimen. Residual stress field of interest is formed by a superposition of initial welding-induced field and secondary stress field caused by plastic deformation of the specimen. A set of actual fringe patterns, which corresponds to a wide variety of residual stress components both ratio and sign, are reconstructed and presented as illustrations. A series of reference fringe patterns is simulated for the most typical cases inherent in residual stress field under study. It is shown that actual interferograms obtained belong to three main groups depending on a typical form of fringes configuration. On this base the main principles of creating the general catalogue of fringe patterns are established and the first version of this catalogue, which is related to reflection hologram interferometry, is developed. A structure of the catalogue that consists of both actual interferograms and reference fringe patterns is described. Possible ways of further catalogue completing and its direct implementing in the course of quantitative determination of residual stresses are discussed. It is shown that both experimental and numerical data aggregated into the first version of the catalogue can be effectively used for a verification of various coherent optics techniques with respect to a determination of residual stress components by means of hole drilling. An analysis of capabilities of reflection hologram interferometry in the field of residual stresses determination comparing with dual-beam speckle-interferometric techniques is presented. Superimposed residual stress field is quantitatively described in detail for both specimen sides of dimensions 260×80 mm². It is shown that fine nuances inherent in residual stress distributions over different specimen faces can be reliably derived from recorded fringe patterns of any type. This study serves as an example of residual stress components determination in real structure with a type of residual stress field to be investigated is unknown before the experiment.     

On the use of acoustic birefringence to determine components of plane stress

To assess the integrity of a structure containing a (known) flaw, it is necessary to know the stresses acting on the flaw. Many common structural elements (e.g. beams and plates) are subjected to either true plane stress or generalized plane stress. For either of these cases, there are three in-plane stresses (one shear stress and two normal stresses) to be determined in general.In this paper, we consider the application of the acoustic birefringence method to generalized plane stress states, where the thickness-averaged values of the shear and normal stresses are sought. It is shown that the times-of-flight are thickness-averaged effects, whereas the polarization directions depend upon local values of stress. Consequently, when the symmetric (axial) component of the stresses dominates the antisymmetric (bending) component, the acoustic birefringence method can be used to determine all three stresses, if the boundary conditions are known. For unknown boundary conditions, the normal stresses can be determined to within arbitrary functions.Problems arise in using the normal incidence technique when either the shear stress vanishes, or the symmetric (membrane) components of generalized plane stress do not dominate the bending components. In the former case, the thickness-averaged difference in normal stresses can be obtained, provided that the birefringence in the unstressed state is known. In the latter case, the (averaged) difference in normal stresses can be obtained by measuring time-of-flight differences of off-axis SH-waves propagating in the planes of material symmetry of the plate.     

Automation of a neutron diffractometer for analysis of residual stress inside complex engineering components

Residual stress measurement using neutron diffraction is becoming an increasingly important tool in engineering stress analysis. To this effect, a new generation of dedicated engineering strain instruments are being built at neutron sources, offering considerable improvements in both counting time and spatial resolution. Alongside these improvements, measurements in complex geometry prototype components are increasingly in demand. As a result, there is a strong drive towards integrated sample positioning systems that allow for simplified setup and operating of experiments on components with complex geometries.     

The Wall Stresses and Insert Load in a Two‐Dimensional Flat‐bottomed Bin with an Equilateral Triangle Insert

The wall stresses and insert load in a two‐dimensional flat‐bottomed bin with a flow corrective insert were investigated. The static wall stress distributions produced by the granular solids were measured and compared with the theoretical prediction using the differential slice method. The variations in the dynamic wall stresses and the dynamic response of the insert load with time were obtained. The comparison of the experimental insert load to the theoretical prediction was demonstrated. In addition, the effect of the flow corrective insert upon the wall stress and insert load was investigated. As the insert half‐angle increases, the effect of disrupting the contact force network above the insert decreases, and the insert load produced by the granular solids increases. Employing the results obtained using stress measurements, the pulsation phenomena of wall stress and insert load in a bin with a triangle flow corrective insert may be further understood.     

Surface deformation induced by a variation in surface stresses in anisotropic half-regions

The deformations and the stresses in anisotropic half-regions taking into account surface stresses originating from surface energy, which exists originally at surfaces and interfaces dividing phases, are analysed theoretically. In the present paper, the equilibrium equation of force considering surface stresses is used to calculate the inelastic deformation induced by a variation in surface stresses. The problem of varying surface stresses in a half-surface of a half-infinite anisotropic domain is analysed using the theory of elasticity. This problem is related to the occurrence of cracks in contaminated, oxidized or chemisorbed surfaces. Stress analysis on the basis of continuum mechanics is performed precisely under the boundary condition taking into account surface stresses. The Fourier transform technique is applied to perform the analysis, and the components of stress and displacement are expressed in an explicit form. The shear component of bulk stress attains infinity at the edge of discontinuity of the surface stresses, and the free surface deforms like an edge dislocation. This result suggests that cracking in a chemically contaminant surface is easier than in a clean surface.     

Nature of the structural degradation of rail surfaces during operation

Patterns in the transformation of the structural and phase states and the defect substructure of rail surface layers up to 10 mm thick during long-term operation (gross transit tonnages of 500 and 1000 mln t) are found via optical, scanning, and transmission electron microscopy. According to the nature of the fracture and the degree of defectiveness, three layers can be distinguished: a surface layer, a transition layer, and the base metal. It is shown that the operation of steel rails is accompanied by full fractures in surface layers up to 15 μm thick with lamellar pearlite grains and the formation of ferrite–carbide mixtures with nanosized particles. The deformation of steel increases the densities of scalar and excess dislocations, the curvature–torsion values of the crystal lattice, and the amplitudes of internal stress fields. Structural elements that can act as stress concentrators are identified.     

Tensor field tomography of residual stresses

An approximate method is proposed for determining residual stresses in elonagted transparent articles featuring weak variation of the stress field along the axis. The proposed method is a generalization of the well-known method of determining internal stresses in optic fibers based on the integrated photoelasticity measurements. Complete determination of the stress tensor components is performed within the framework of the concept of temperature-dependent residual stresses.     

An analysis of residual stress patterns resulting from hole expansion in an infinite plate, a thick cylinder, and an asymmetric lug

Residual stresses were induced in three specimen geometries: a quasiinfinite plate, a thick cylinder and an asymmetric lug. In each case, a hole expansion process was used, whereby the bore was expanded into the plastic regime; this in effect left residual compression at the bore and residual tension in the far field. In view of the symmetry, the stress patterns in the quasi-infinite plate were measured by a hole drilling method, using an interferometric moiré method to measure the resulting strain patterns. In the case of the thick cylinder and the asymmetric lug, the residual stresses were evidenced by a dissection method. A comparison with theoretical treatments shows that the theory predicts an approximate upper bound to the actual stress levels in the quasi-infinite plate. In the lug geometry, there was a similar systematic difference between theory and experiment.